Field of the Invention
The invention relates to a method for operating a combined-cycle power plant, in which heat contained in an expanded working medium of an associated gas turbine operable with both gas and oil as fuel is utilized in order to generate steam for an associated steam turbine having at least one high-pressure stage. The invention also relates to a combined-cycle power plant which is particularly suitable for carrying out the method, including a gas turbine operable with both gas and oil as fuel and having a waste-heat steam generator downstream of the gas turbine on the flue-gas side for generating steam for an associated steam turbine having at least one high-pressure stage.
In a combined-cycle power plant, the heat contained in the expanded working medium from the gas turbine is utilized in order to generate steam for the steam turbine. Heat transmission takes place in a waste-heat steam generator which is downstream of the gas turbine and in which heating surfaces in the form of tubes or tube bundles are disposed. Those heating surfaces, in turn, are connected into a water/steam circuit of the steam turbine. The water/steam circuit includes one or more, for example two or three, pressure stages. Each pressure stage conventionally has a preheating heating surface (economizer), an evaporator heating surface and a superheater heating surface. Depending on the pressure conditions prevailing in the water/steam circuit of the steam turbine, a thermodynamic efficiency of about 50% or more is achieved through the use of a combined-cycle power plant of that type which is known, for example, from European Patent 0 148 973 B1.
The gas turbine of a combined-cycle power plant of that type may be constructed to operate with different kinds of fuel. However, the requirements placed on the waste-heat steam generator downstream of the gas turbine on the flue-gas side are different, depending on the type of fuel on which the construction is based. For example, gas used as a fuel for the gas turbine normally has high purity, so that flue gas flowing out of the gas turbine contains only small amounts of impurities.
In contrast thereto, if the fuel for the gas turbine is fuel oil, impurities in the flue gas flowing out of the gas turbine are to be expected. In that case, in particular, sulfur dioxide (SO.sub.2) or sulfur trioxide (SO.sub.3) may occur, which, after reacting with water in the form of sulfuric acid (H.sub.2 SO.sub.4), may settle on the heating surfaces in the waste-heat steam generator and attack them. The requirements placed on the waste-heat steam generator when oil is used as a fuel for the gas turbine must therefore be different from those when gas is used as the fuel for the latter.
In particular, when oil is used as a fuel for the gas turbine, it is necessary to ensure that the heating surfaces connected into the water/steam circuit of the steam turbine and the line components inside the waste-heat steam generator are at a sufficiently high temperature, namely a temperature above the dew point of sulfuric acid. For that purpose, when the gas turbine operates with oil, the inlet temperature of the water or condensate flowing into the waste-heat steam generator is raised, as compared with the gas turbine operating with gas, and is set at about 120.degree. to 130.degree. C.
A combined-cycle power plant, in which fuel oil is only provided as a fuel for the gas turbine for a brief operating period, for example for 500 to 1,500 h/a, as a "back-up" to natural gas, is usually constructed and optimized primarily for the gas turbine to operate with natural gas. In order to ensure that, when the gas turbine operates with fuel oil, the condensate flowing into the waste-heat steam generator has a sufficiently high inlet temperature, the necessary heat may be extracted from the waste-heat steam generator itself in various ways.
One possibility is to bypass a conventionally provided condensate preheater completely or partially and to heat the condensate by the supply of low-pressure steam in a feed-water tank connected into the water/steam circuit. However, at low steam pressures, such a method necessitates a large-volume and possibly multi-stage heating steam system in the feed-water tank and, in the case of long heating-up periods, that may put at risk a deaeration function which normally takes place in the feed-water tank.
In order to ensure effective deaeration of the condensate, the condensate temperature in the feed-water tank must always be maintained in a temperature range of between 130.degree. and 160.degree. C., and the heating-up period of the condensate in the feed-water tank should be kept as short as possible. That may be carried out, for example, by preheating the condensate through an additional preheater heated through the use of steam.
In order to provide sufficient heat for that purpose, in the case of two-pressure or three-pressure plants it is often necessary to extract hot water from a high-pressure economizer of the waste-heat steam generator. However, the disadvantage thereof, particularly in the case of three-pressure plants, is that the delivery of a normally provided high-pressure feed pump may be influenced, and that the additional condensate preheater has to be constructed in a particularly inefficient way for the high pressure and large temperature differences.
Furthermore, in the case of fuel-oil operation, throttle losses of the feed pump or each feed pump occur adversely. Moreover, the extraction of hot water from the high-pressure economizer leads to a reduction in the high-pressure steam quantity due to a lowering of a so-called high-pressure approach temperature, thus in turn leading to a reduction in plant efficiency.
Another proven method is, when the gas turbine operates with oil, to assist the heating-up of the condensate in the feed-water tank or in the deaerator through the use of steam extracted from an intermediate superheater line. However, that method cannot be employed in the case of plants without a feed-water tank or without a deaerator.
The above-mentioned concepts of condensate preheating when oil is used as a fuel for the gas turbine are complicated in view of the components which are required as well as in view of the operating mode of the combined-cycle power plant. Moreover, plant efficiency is only limited when the gas turbine operates with oil.